1. Prior Art
As U.S. Pat. No. 4,441,082 discloses a switched capacitor automatic gain control loop utilizing MOS technology and providing an AGC circuit integrated on a monolithic IC chip. U.S. Pat. No. 4,441,082 discloses a rectification circuit and converting analog signals to drive the counter.
2. Background
Automatic gain control (AGC) circuits are widely used in a variety of applications for maintaining a constant signal output levels for varying amplifier input signal levels. A typical electronic amplifier accepts an incoming electronic signal, such as an audio tone, amplifies that signal by a multiplying gain factor determined by the amplifier stage, and provides an output signal that is an amplified replication of the input signal.
It is an inherent characteristic of such an amplifier to respond linearly to the amplitude of the applied input signal. Because the amplifier stage maintains a fixed gain, both weak incoming signals and strong incoming signals are multiplied by the same gain factor such that the output of the amplifier stage responds correspondingly to the input signal. Weak (i.e., small amplitude) components of the input signal, although amplified, will form small amplitude components of the output signal. The overall result is that the amplifier output signal varies in amplitude in accordance with the amplitude variance of the input signal. As a result, in the particular example of a tone amplifier, the listener will experience a fading effect as the tone varies in loudness according to the amplitude of the signal.
An Automatic Gain Control ("AGC") system eliminates this fading effect by maintaining a constant level of amplitude at the output of the amplifier stage by varying amplifier stage gain. In an AGC circuit the gain of an amplifier stage varies inversely to the amplitude of the input signal level. This is accomplished by setting the output level of the amplifier initially to a predetermined reference level. The AGC circuitry seeks to maintain this reference level. When the amplitude of the input signal decreases below a predetermined threshold level, the AGC circuit senses the decrease in amplitude at the output of the amplifier. It thus increases the amplifier stage gain until the amplitude of the output signal increases to the reference level. If the amplitude of the input signal increases above the nominal level, the AGC circuit senses the increase in amplitude at the output of the amplifier stage, and decreases the amplifier stage gain until the amplitude of the output signal decreases to the reference level. The AGC circuit provides the necessary control signals that control the gain setting of amplifier stages.
FIG. 1 illustrates one of the most common embodiments of a prior art AGC circuit. The conventional AGC system such as that shown in FIG. 1, however, cannot be suitably adapted for use in a metal-oxide-semiconductor (MOS) implementation. This is due to the fact that a gain setting variable resistor is an inaccurate passive device in MOS technology. As a result, variable resistive dividers embodied in a MOS device is likely to cause high harmonic distortion when pressed for a large dynamic performance. The large time constants often needed for the attack and release times, (in the order of milliseconds), require passive components of large physical dimensions. This precludes their placement on a large scale integrated circuit (IC) chip.
Further, operational amplifiers realizable in MOS technology do not provide sufficiently high gain or low offset voltage for most desired applications. They also tend to possess non-linear distortion characteristics. The output amplitude is subject to process temperature variations and not well controlled. As a result, prior art implementations of AGC circuits are generally unsatisfactory for MOS technology without utilizing complex circuitry at considerable cost.